Recently, as explosive growth in radio data traffic has expected due to activation of an echo system related to wireless Internet such as a smart phone, App store, etc., interest in frequency sharing as a method for expanding frequency supply has increased correspondingly. However, problems can arise in that it can be difficult to rapidly respond to such increase and frequency supply can be limited under a current frequency management system. That is, a process of recovering or rearranging existing frequencies and distributing the frequencies to new service providers via auction or the like can be ineffective in terms of time and it becomes more difficult to ensure new frequencies on a frequency distribution table basically.
A frequency sharing method has been in the spotlight as a scheme capable of solving the above problems. Advocates for frequency sharing consider that a current shortage of frequencies is caused by a conventional partition type frequency management method and can be solved via a sharing method although a frequency looks deficient on the frequency distribution table. However, frequency sharing is not a new concept but a method for resource management of a communication system in terms of technology, and various frequency sharing methods have been applied. Examples of the frequency sharing method may also include a cellular method, time division multiple access (TDMA), code division multiple access (CDMA), etc. that are conventionally and widely used.
According to an existing sharing method, interference can be effectively suppressed under the same technological standard or control of service providers. However, a distributional sharing method that has been recently introduced, for example, cognitive radio (CR) likely to cause tragedy of commons and harmful interference due to indiscriminate use of frequencies. Accordingly, the method can cause problems in terms of frequency management and protection of existing users and contradict with the intent of a market-based frequency policy as the current tendency of a frequency management system.
However, advocates for frequency sharing consider that the problems in terms of protection of existing users from interference and the problems due to indiscriminate use of frequencies can be overcome or controlled by political and technological means such as technological regulation, standard establishment, etc.
A type of frequency sharing method can be classified according to a sharing ‘method’ and ‘subject and object’. Here, in detail, coexistence and cooperative methods are considered as the sharing method. In addition, the sharing subject and object can be classified into sharing among equals in which a subject and an object have equivalent positions and primary-secondary sharing in which a primary user and a secondary user share a frequency.
First, in the cooperative method, a separate protocol is present, and thus, resource allocation and interference avoidance are possible via organic cooperation among all users. Resource allocation and interference avoidance can be centrally performed via a base station (BS), etc. or may be distributively performed between users. Examples of the former may include a mobile communication system such as CDMA/TDMA methods and examples of the latter may include an ad-hoc system.
On the other hand, a case in which users separately perform interference avoidance without resource allocation using this common protocol can be referred to as mutual coexistence. Examples of the mutual coexistence (or simply coexistence) scheme may include a wireless local area network (LAN), a cordless phone, etc. as currently commercialized technologies and may also include overlay and underlay technologies. Compared with the cooperative scheme, according to the coexistence scheme, resource allocation and interference control are passively intervened in terms of technologies, and thus, problems arise in that the probability of interference increases.
In addition, a coexistence topology may be a centralized coexistence topology and a distributed coexistence topology. The centralized coexistence topology has global information for decision making and thus effectively manages interference. However, according to the centralized coexistence topology, performance is degraded due to excessive computational overhead in proportion to the number of devices to be managed. On the other hand, the distributed coexistence topology requires low computational and communication overhead. However, compared with centralization approach, according to the distributed coexistence topology, local optimization is used, and thus, high interference can be caused.